The present invention relates to a light emitting diode which has high emission output and a process for fabricating the same.
In general, in order to increase emission output of a light emitting diode, there is a method of enhancing internal quantum efficiency which is efficiency to convert electricity to light or a method of enhancing light output efficiency which is efficiency to take out light from the light emitting diode.
As means for enhancing internal quantum efficiency, single hetero structure and double hetero structure have been already utilized. According to these structures, internal quantum efficiency is significantly improved to reach a high output.
On the other hand, though the light output efficiency is a major factor to determine the emission value of the light emitting diode, to obtain high efficiency in the light output is difficult since light is reflected on the surface of the chip due to the difference between the refraction index of the light output surface of the chip of the light emitting diode and that of the external(air).
Therefore, a light emitting diode in which light going to the back surface is reflected on the back surface and is taken out from the front surface is suggested. The light emitting diode is obtained by growing an AlGaAs thick film on a GaAs substrate, growing an AlGaAs layer as a light emitting portion thereon, and the removing GaAs substrate.
In order to effectively reflect the light on the back surface, it is needed to prevent the light from being absorbed into an epitaxial layer or the substrate. This idea is well known in the light emitting diode of GaP, InP and the mixed crystal thereof. In a GaAs substrate, it is needed to use an AlGaAs substrate which has AlAs mixed crystal of relatively high ratio since light is absorbed into the GaAs substrate. Therefore, as described above, the AlGaAs layer of thick film is grown to be used in place of a single element substrate and the substrate originally used is removed.
As mentioned above, in order to make an AlGaAs epitaxial wafer which has a structure in which light is reflected on the back surface of the chip of the light emitting diode, the AlGaAs substrate is needed. It is obtained by growing an AlGaAs thick film on the GaAs substrate, growing an AlGaAs layer as a light emitting portion thereon, and removing the GaAs substrate. The AlGaAs thick film is in general required to be around 50 micrometers thick so as to hold the shape of the wafer for the chip of the light emitting diode.
However, when an AlGaAs layer having high ratio of mixed crystal is epitaxially grown by the gradually cooling method, it is difficult to grow an AlGaAs epitaxial layer which is thick and high in ratio of AlAs mixed crystal since the amount of Al solved is large whereas that of GaAs solved is small when GaAs and Al are added to a solvent of Ga. Because of this, it is necessary to take means or method of increasing the growth starting temperature to around 1000.degree. C. (though normally approximately 900.degree. C.), thickening the thickness of a solution reservoir, and slowing down the cooling rate or the like. In these methods, however, the amount of raw material is increased, the growth takes a long time and the cost for manufacturing it goes up. Besides, the film thickness within the surface is dispersed to the extent of 50 to 100 micrometers, it is difficult to improve the uniformity in the film thickness and the properties, and the wafer becomes brittle or to the internal distortion caused by the dispersion in the film thickness.
There is the temperature difference method in addition to the gradually cooling method, but in this method the number of the epitaxial wafer which can be grown in one step is very few, i.e., around one or two. Therefore, when the epitaxial wafer is mass-produced, a number of growth devices are needed and the yield becomes low due to the dispersion of the properties, and the cost for manufacturing it goes up. Based upon the above reasons, a light emitting diode reflecting the light without the AlGaAs thick film to improve the efficiency has been desired.